The standard system used today for treating soils contaminated by petroleum hydrocarbons are known as biopiles. Biopiles, also known as biocells, bioheaps, biomounds, and compost piles, are used to reduce concentrations of petroleum hydrocarbons in excavated soils through the use of biodegradation. This technology involves heaping contaminated soils into piles (or “cells”) and stimulating aerobic microbial activity within the soils through the aeration and addition of minerals, nutrients, and moisture. The enhanced microbial activity results in degradation of adsorbed petroleum-product constituents through microbial respiration.
Biopiles are similar to landfarms in that they are both above-ground, engineered systems that use oxygen, generally from air, to stimulate the growth and reproduction of aerobic bacteria which, in turn, degrade the petroleum hydrocarbons adsorbed in the soil. While landfarms are aerated by tilling or plowing, biopiles are aerated by forcing air to move by injection or extraction through slotted or perforated piping placed throughout the pile.
Biopiles have been proven effective in reducing concentrations of nearly all the constituents of petroleum products typically found at storage tank sites. Lighter (more volatile) petroleum products like gasoline tend to be removed by evaporation during aeration processes (i.e., air injection, air extraction, or pile turning) and, to a lesser extent, degraded by microbial respiration. The mid-range hydrocarbon products (e.g., diesel fuel, kerosene) contain lower percentages of lighter (more volatile) constituents than does gasoline. Biodegradation of these petroleum products is more significant than evaporation. Heavier (non-volatile) petroleum products (e.g., heating oil, lubricating oils) do not evaporate during biopile aeration; the dominant mechanism that breaks down these petroleum products is biodegradation.
The typical height of biopiles varies between 3 and 10 feet. Additional land area around the biopile will be required for sloping the sides of the pile, for containment berms, and for access. The length and width of biopiles is generally not restricted unless aeration is to occur by manually turning the soils. In general, biopiles which will be turned should not exceed 6 to 8 feet in width. To prevent possible leaching of contaminants from the biopile into the underlying groundwater, biopiles require to be constructed on top of an impermeable liner. Leachate that drains from the biopile is then collected for treatment and disposal. Biopiles are also typically covered by a semi-permeable liner to allow air circulation but minimize infiltration of excess water from precipitation.
Because volatile constituents tend to evaporate from the biopile into the air during extraction or injection, rather than being biodegraded by bacteria, capture or containment of vapors is required. If air is extracted from the pile by applying a vacuum to the aeration piping, volatile constituent vapors will pass into the extracted air stream which can be treated typically through a biofilter and/or carbon adsorption. Hair is injected into the pile, from the bottom up, extraction piping on top of the pile will recover the volatiles for treatment by filtration.
Biopile construction is an effective method for treating large amounts of contaminated soils. However biopile construction is less adapted for treating smaller amounts of contaminated soils as it requires heavy machinery to build up the biopile and install piping and requires trained personnel for the aeration process and the addition of minerals, nutrients, and moisture and to monitor the biopile sites. Temporary and project-specific biopiles are usually underlain by a fragile geomembrane liner which will typically not be reused on another biopile project. Bottom piping is also subject to damage from heavy equipment during soil mixing operations and will typically not be reused on another biopile project. At permanent soil biotreatment facilities the bottom surface is typically constructed of concrete which may be costly. For small or medium size spills in small communities or remote communities, biopile construction, maintenance and monitoring is an expensive proposition and is often not feasible for lack of equipment, materials and trained personnel.
Therefore, there is a need for a system and apparatus for treating soils contaminated by petroleum hydrocarbons such as gasoline, diesel and heating oil and soils contaminated by chlorinated hydrocarbons and volatile organic compounds which adapted for small or medium size spills in small communities or remote communities.